The present invention relates to processing a charge of a solid material to heat the solid material.
The present invention relates particularly, although by no means exclusively, to processing a charge of a solid material which has low thermal conductivity under conditions including high temperature and pressure.
The present invention relates more particularly to:
(i) upgrading carbonaceous materials, typically coal, under conditions including high temperature and pressure to increase the BTU value of the carbonaceous materials by removing water from the carbonaceous materials; and
(ii) cooling the heated carbonaceous materials.
U.S. Pat. No. 5,290,523 to Koppelman discloses a process for upgrading coal by the simultaneous application of temperature and pressure.
Koppelman discloses thermal dewatering of coal by heating coal under conditions including elevated temperature and pressure to cause physical changes in the coal that results in water being removed from the coal by a xe2x80x9csqueezexe2x80x9d reaction.
Koppelman also discloses maintaining the pressure sufficiently high during the upgrading process so that the by-product water is produced mainly as a liquid rather than as steam.
Koppelman also discloses a range of different apparatus options for carrying out the upgrading process. In general terms, the options are based on the use of a pressure vessel which includes an inverted conical inlet, a cylindrical body, a conical outlet, and an assembly of vertically or horizontally disposed heat exchange tubes positioned in the body.
In one proposal to use a Koppelman-type apparatus, the vertically disposed tubes and the outlet end are packed with coal, and nitrogen is injected to pre-pressurise the tubes and the outlet end. The coal is heated by indirect heat exchange with oil that is supplied as a heat transfer fluid to the cylindrical body externally of the tubes. Further heating of the coal is promoted by direct heat exchange between the coal and steam which acts as a working fluid within the packed bed. In addition, the steam pressurises the tubes and the outlet end to a required pressure.
The combination of elevated pressure and temperature conditions in the tubes and the outlet end evaporates some of the water from the coal and thereafter condenses some of the water as a liquid. A portion of the steam generated following the addition of water also condenses as a liquid in colder regions of the tubes due to the elevated pressure. Steam which is not condensed, and which is in excess of the requirements for optimum pressurisation of the packed bed, must be vented. In addition, non-condensable gases (eg CO, CO2) are evolved and need to be vented. Periodically, liquid is drained from the outlet end.
Finally, after a prescribed residence time, the vessel is depressurised and the upgraded coal is discharged via the outlet end and subsequently cooled.
The above described proposal to use a Koppelman-type apparatus requires the use of oil as a heat transfer fluid at close to its operating temperature limit. This is undesirable from environmental and occupational health viewpoints. Other high temperature liquids such as molten salt or molten metal may be used as alternatives but these also have limitations in use.
In another proposal to use a Koppelman-type apparatus, steam rather than oil is used as a heat transfer fluid in direct rather than indirect contact with coal. The disadvantages of this proposal include limited options to scale up to a commercial plant size and difficulties in controlling heating rate.
An object of the present invention is to provide an improved method and apparatus for upgrading coal by the simultaneous application of temperature and pressure which does not rely on the use of oil as the heat transfer fluid.
According to the present invention there is provided a method of heating a solid material in a process vessel, which method comprises:
(a) supplying a charge of the solid material to the vessel to form a packed bed;
(b) supplying a fluid to the packed bed to pressurise the contents of the vessel;
(c) supplying steam to the vessel to heat the solid material in the packed bed by indirect heat exchange while maintaining the contents of the vessel under pressure; and
(d) controlling the operating conditions in step (c):
(i) to transfer heat to the solid material and allow water in the solid material to be removed as a liquid phase in a first xe2x80x9cwetxe2x80x9d stage of the method; and
(ii) to transfer heat to the solid material to boil at least a part of the remaining water from the solid material as a vapour phase in a second xe2x80x9cdryxe2x80x9d stage of the method.
The term xe2x80x9coperating conditionsxe2x80x9d is understood to mean any conditions which have a bearing on the heating of the solid material and the removal of water from the solid material and includes, by way of example, operating conditions such as steam pressure, steam temperature and steam flow rate which influence the temperature in the packed bed.
It is preferred that step (d) comprises controlling the operating conditions so that a substantial portion of the steam condenses during indirect heat exchange with the solid material in the packed bed in the wet phase of the method.
It is preferred particularly that step (d) comprises controlling the operating conditions so that at least 80% of the steam condenses during indirect heat exchange with the solid material in the packed bed in the wet phase of the method.
It is preferred that the wet stage of the method heats the solid material to a temperature of the order of 250xc2x0 C.
It is preferred that the dry stage of the method includes:
(i) a xe2x80x9cdwellxe2x80x9d part during which the remaining water that is removed in the dry stage boils from the solid material; and
(ii) a subsequent heating part during which the solid material is heated to a final temperature.
It is preferred that the final temperature of the solid material in the dry stage be on average in the range of 270 to 420xc2x0 C. to ensure optimum upgrading of the solid material.
In order to achieve temperatures of at least 270xc2x0 C. in the dry stage, it is preferred that the method comprises supplying superheated steam during the dry stage of the method.
It is preferred particularly that step (d) comprises controlling the operating conditions so that the pressure of the superheated steam in the dry stage of the method is greater than the pressure in the packed bed so as to promote boiling of water in the packed bed.
Typically, step (d) comprises controlling the pressure of the steam in the wet stage relative to the pressure in the packed bed so as to control the condensing temperature of the steam to be less than that of the boiling temperature of water in the packed bed. This step ensures operation which avoids boiling of water exuded from the solid material in the packed bed during the wet stage of the method.
It is preferred that the method comprises:
(a) supplying superheated steam to a first process vessel to heat solid material in the packed bed in the first vessel by indirect heat exchange during the dry stage of the method;
(b) supplying steam discharged from the first process vessel to a second process vessel to heat solid material in the packed bed in the second vessel by indirect heat exchange during the wet stage of the method.
The above described use of two (or more) process vessels with separate charges of solid material is particularly advantageous because it makes use of steam in a superheated state in the dry stage to heat the solid material in the packed bed to temperatures to boil water from the solid material and to further heat the solid material to a final temperature and thereafter makes use of steam in the wet stage to heat solid material without boiling the water in the solid material.
It is preferred particularly that the method further comprises:
(a) discharging heated solid material from the first vessel after completing the wet and dry stages of the method and removing a required level of water from the solid material during these stages;
(b) filling the first vessel with solid material and pressurising the contents of the vessel; and
(c) changing the flow of steam so that the superheated steam flows first through the second vessel to heat the solid material in the packed bed by indirect heat exchange in the dry stage of the method and the steam discharged from the second vessel flows through the first vessel and heats solid material in that vessel by indirect heat exchange in the wet stage of the method.
It is preferred more particularly that the method comprises repeating the above described sequence of steps of emptying and filling the vessels and changing the flow of steam through the vessels.
According to the present invention there is also provided an apparatus for heating a solid material which comprises:
(a) a process vessel for containing a packed bed of the solid material; and
(b) a heat exchange circuit for supplying steam to the process vessel to heat the solid material in the packed bed via indirect heat exchange, which heat exchange circuit comprises:
(i) a heat exchange assembly in the process vessel, which assembly comprises a passageway for steam and a plurality of heat exchange surfaces which, in use, extend into the packed bed;
(ii) a condenser for condensing steam discharged from the heat exchange assembly; and
(iii) a boiler for generating steam for the heat exchange assembly from the water condensed in the condenser.
It is preferred that the exchange circuit further comprises a means for storing steam to allow for variations in flow and pressure during normal operating conditions, load/unload, start-up and shut-down.
It is preferred that the apparatus comprises two or more process vessels for containing packed beds of the solid material.
With this arrangement, it is preferred that the heat exchange circuit comprises one of the heat exchange assemblies in each of the vessels and that the heat exchange assemblies be connected together so that steam can flow in series or in parallel through the heat exchange assemblies.